The use of heliostats in the field of concentrating solar power (CSP) is well established in the prior art. A typical CSP system includes a centralized tower and a plurality of ground mounted heliostats. The centralized tower serves as the focal point onto which individual heliostats redirect sunlight. The concentration of sunlight at the tower focus increases, as a general statement, with the number of heliostats. The high concentration of solar energy is converted by the tower into other useful forms, typically heat which can then be used either directly or be used to generate steam to power electrical generators. It should be noted that it is also possible to convert the solar energy directly into electricity through the use of any number of photovoltaic devices generally referred to as solar cells.
Heliostats generally include one or more mirrors to redirect sunlight, support structure to hold the mirror(s) and to allow the mirror(s) to be articulated, and actuators such as motors to effect the articulation. At a minimum, heliostats must provide two degrees of rotational freedom in order to redirect sunlight onto a fixed tower focus point. Heliostat mirrors are generally planar, but could possibly have more complex shapes. Heliostat articulation can follow an azimuth/elevation scheme by which the mirror rotates about an axis perpendicular to the earth's surface for the azimuth and then rotates about an elevation axis that is parallel to the earth's surface. The elevation axis is coupled to the azimuth rotation such that the direction of the elevation axis is a function of the azimuth angle. Alternatively heliostats can articulate using a tip/tilt scheme in which the mirror rotates about a fixed tip axis that is parallel to the earth's surface. The tilt axis is orthogonal to the tip axis but its direction rotates as a function of the tip axis. The tilt axis is parallel to the earth's surface when the heliostat mirror normal vector is parallel to the normal of the earth's surface.
Most heliostats themselves and systems or collections thereof are controlled by computer control systems. For example, a computer can be provided with a latitude and longitude of the heliostat's position on the earth along with the time and date at that location. Using this information along with known planetary movement information, a control computer can calculate the direction of the sun as seen from the mirror, e.g. its compass bearing and angle of elevation. Then, given the direction of the target, a control computer can calculate the direction of the required angle-bisector. Based upon this data, the control computer can send control signals to tip and tilt controls, such as including drive systems, often utilizing stepper motors, as conventionally known for turning the mirror to the correct alignment. This sequence of operations is then normally repeated frequently to keep the mirror properly oriented.
A single heliostat of certain prior art systems can be of a size of about two square meters or greater. A conventional design for a heliostat's reflective components utilizes what is known as a second surface mirror. A sandwich-like mirror structure classically includes, inter alia, a steel structural support, a layer of reflective silver, and a top protective layer of glass. Such a heliostat is often referred to as a glass/metal heliostat. Alternative designs incorporate recently developed adhesives, composite materials, and thin film designs to bring about stronger and lighter materials to reduce costs and weight. Some examples of alternative reflector designs are silvered polymer reflectors, glass fiber reinforced polyester sandwiches (GFRPS), and aluminized reflectors. Problems with conventional designs of heliostats and mirrors arise from the size and mass of such structures as they are to be accurately controlled. The greater the mass of all components along with normal reflector sizes, generally means the more robust the supporting and controlling components must be as such components may also require precision machining and manufacturing. Such qualifications increase expense in the utilization of stronger materials and in the design needs for accurate control of all movements by precision drive mechanisms and associated controls.